From the Archives: How Many Species?

Feb 07 2008 Published by under Biology, Science

This is a highly modified version of a post that appeared back at my old blog quite some time ago. Since it involves a quiz of sorts, I'm not going to post the link back to the original right now. The post with the "answers" will appear on Monday, also slightly modified from the original.

Taxonomy and systematics are the areas of biology that are involved in describing groups of organisms and determining how they relate to one another. One of the jobs associated with these disciplines involves trying to figure out whether or not two different populations of organisms should be considered to be part of the same species. Sometimes this is an easy job - it's pretty clear, for example, that polar bears and penguins are very different sorts of thing. Other times, it's a very hard job. The example I'm going to give you in this post is a difficult case, but a real one. I'll give you the details, and you can take your best stab at the question. On Monday, I'll tell you what the "official" view is.

There are three populations of a flying insect. These populations are physically separated from each other by areas of inhospitable terrain, and members of the populations are not known to come into contact with each other in the wild. Population A is found about 15 km to the northwest of Population B; Population B is found about 50 km to the northwest of Population C. In the past, the areas occupied by Populations A and B were closer together, and may actually have formed a single area. The area where Population C lives was never in contact with the other two populations.

There are no apparent differences between Population A and Population B in either physical appearance or in a specific reproductive behavior. Population C has legs that are a different color from the legs in the other two populations, but is otherwise identical in appearance. The reproductive behavior seen in Population C is very different from that seen in Populations A and B.

The organisms were captured and bred in the laboratory. Experimental crosses were made for the different combinations of these three populations, with the following results:

Table 1: Fertility of male hybrid offspring. The origin of the father is shown in red; the origin of the mother in blue. Fertility was measured as the percentage of male offspring with motile sperm.

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Table 2: Viability of female hybrid offspring. "Normal" indicates that the cross produced roughly equal numbers of male and female offspring. "Reduced" indicates that the cross produced a very small number of viable females. Origin of father in red; origin of mother in blue.

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My question for you is this: How many different species should these three populations be grouped in, and why? I'll post both the "official" answer and some more information about these animals on Monday.

5 responses so far

  • Eric says:

    Well.. I would say two. Group A and B together as a species, and C separately. When C mates with either A or B the male offspring is sterile most of the time and when a mother from C and a father from A or B mate the female offspring is reduced, so it seems C would be considered a different species. However, the viability of offspring doesn't seem to vary much at all whether the mother and father are both from A or both from B, or a mother and father from one of each.
    Then again.. that seems pretty obvious, so I'm probably wrong.

  • Eric says:

    Without any other evidence (DNA for instance) I would say two species are now involved. A & B are morphologically and behaviorally the same and reproductively compatible (almost meta-populations within a larger population). C has not contact, and was never in contact, has morphological and behavioral differences as well as infertile male and reduced fertility female offspring. But as Eric said above...it seems too easy to be right.

  • Bob O'H says:

    Doesn't the answer depend on whether you're a splitter or a lumper?
    Based only on this evidence, populations A and B can be grouped into a single species. Population C males can produce viable females, and viable males with population A. We're not told anything about the abilities of these hybrids to cross, so they could be infertile, in which case population C is a separate species. Or they could be totally fertile, in which case they might be lumped into the same species as populations A and B (as there gene flow is possible between the two).
    Of course, one could just wait a couple of millennia for full reproductive isolation to occur. Then there's no problem.
    Bob

  • JS says:

    Depends on your definition of species. Based only on these data, I would call A and B sub-populations of the same species The difference between the (3,1) and (3,2) elements in the first matrix may be an artifact, depending on the sample size and experimental setup, and at any rate, I would submit that you could almost certainly find similar examples elsewhere in the biosphere where a sub-population more readily inbreeds with another species than the average for the 'parent' species.
    But the fact that you can get stuff to breed in the lab does not necessarily mean that it'll interbreed in nature. If, say, species A lives in trees on the southern slope of mountains, but only above 300 m, while species B in valleys with high humidity, the fact that they are sufficiently related to interbreed in the lab should not, in my admittedly inexpert opinion, qualify them as being the same species (assuming, of course, that A and B live in different habitats because they would not be viable in the other habitat, not simply because they are separated by - say - mountain range).
    As an aside, I've been reading you long enough to remember when this first went on the air, but I've forgotten the answer in the meantime 😛
    - JS

  • Jim Thomerson says:

    We ran into an interesting situation in Rivulus hartii (a killifish) in Venezuela. R. hartii occurs along the coast of Venezuela and on Margarita Island, as well as Trinidad and Tobago, and maybe Grenada. As we look at specimens going from west to east along the coast we find that in specimens to the west and on Margarita, the males have a colored anal fin and the females have a clear anal fin. Proceeding on to the east we find that the females have colored anal fins and the males have clear anal fins. One suspects an isolating mechanism.
    We did hybridization experiments among various populations (west, east, Margarita, Trinidad). All heterolocality crosses were significantly more successful than control homeolocality crosses; more eggs and more healthy fry. F1 and F2 showed no loss of fertility nor did crosses among the various F1's. However, there is a recent paper out on R. hartii DNA which I have not yet digestd, which finds as much as 11% difference between some populations.